An electric vehicle (EV) and a hybrid electric vehicle (HV) require a high voltage in the range of about 100V to 400V. These vehicles use therefore a combined battery pack that includes the cells of a large number of secondary batteries connected to each other in series. In the case of a combined battery pack having a voltage of 300V, for example, it is necessary to connect the cells of 150 lead batteries each having a voltage of 2V/cell in series, the cells of 250 nickel hydrogen batteries each having a voltage of 1.2V/cell in series or the cells of 80 lithium ion batteries each having a voltage of 3.6V/cell. The lithium ion battery has a characteristic superior to the lead battery and the nickel hydrogen battery in respect of volume energy density, weight energy density and cycle life.
The secondary battery such as the lithium ion battery in particular does not endure excessive charging and excessive discharging. Therefore, the amount of electric charge accumulated in the battery decreases substantially and heat is dissipated considerably, unless the battery is used within a limited voltage range. Thus, to use a combined battery pack, constant-voltage electric-charging control needs be executed to put the voltage appearing between the terminals of the combined battery pack within a voltage range determined by an upper-limit voltage and a lower-limit voltage. In addition, a protection circuit is also required to prevent the voltage appearing between the terminals of the combined battery pack from exceeding the limited voltage range.
Moreover, the combined battery pack has cell voltage dispersions caused by SOC (state of charge) dispersions among cells composing the combined battery pack. In the combined battery pack, the cell SOC and, hence, the cell voltage vary from cell to cell due to the fact that there are differences in storage capacity and differences in self electric discharging characteristic among the cells. In particular, since the lithium ion battery has a very poor characteristic of enduring excessive electric charging and excessive electric discharging in comparison with other types of secondary batteries, variations in SOC from cell to cell may become even worse. In this case, the combined battery pack will not be usable at all.
To solve the above drawbacks, JP 2004-080909A proposes that, when variations exceeding a predetermined level are detected while the vehicle is traveling, the amount of residual electric charge left or available in the combined battery pack is adjusted by electrically charging or discharging each battery cell to a target residual electric-charge amount set for a stop state of the vehicle. It also proposes that, when there is a battery cell with its inter-terminal voltage exceeding a uniform reference voltage in a stop state of the vehicle, electric charge accumulated in the battery cell is discharged so that the voltage appearing between the terminals of each battery cell is made equal to the uniform reference voltage.
In addition, JP2004-248348A proposes that, when a voltage generated by a secondary battery being subjected to an electric discharging process decreases abnormally, an electric discharging switch employed in an electric discharging circuit is turned off forcibly to avoid an excessive electric discharging of the secondary battery.
FIG. 4 shows a comparator employed in a conventional cell voltage equalization apparatus provided for a combined battery pack 1. The combined battery pack 1 has battery cells BC1, - - - , BCn−1, BCn and BCn+1, which are connected to each other in series. On the other hand, a reference voltage generation circuit 2 has a configuration comprising resistors R1, - - - , Rn−1, Rn, Rn+1 and Rn+2, which are connected to each other to form a series circuit. A n-th comparator CPn compares the voltage Vn+1 appearing at the negative-side terminal of a battery cell BCn with a divided voltage VRn+1 generated by the reference voltage generation circuit 2 as a reference voltage. Similarly, a comparator CPn−1 (not shown) compares the voltage Vn appearing at the positive-side terminal of the battery cell BCn with a divided voltage VRn also generated by the reference voltage generation circuit 2 as another reference voltage. An electric discharging process of the battery cell BCn is controlled based on a signal output by the comparator CPn and a signal output by the comparator CPn−1.
The comparator CPn includes PNP-type differential amplifier transistors Q1 to Q4. The base of the transistor Q1 is connected to the negative-side terminal of the battery cell BCn. The voltage Vn+1 appears at the negative-side terminal of the battery cell BCn. The base of the transistor Q2 is connected to the point of junction between the resistors Rn and Rn+1. The divided voltage VRn+1 generated by the reference voltage generation circuit 2 appears at this point of junction. When the voltage Vn+1 becomes higher than the reference voltage VRn+1 by a difference at least equal to a forward-direction voltage Vf, the pn junction between the collector and base of the transistor Q4 is biased in the forward direction, causing the transistor Q4 to terminate an amplifying operation. Thus, when the degree of non-uniformity of cell voltages increases in the cell voltage equalization apparatus, the comparators do not operate normally, making it impossible to execute normal control of the cell voltage equalization process.